Polyolefin-derived carbon fibers containing boron

ABSTRACT

A process includes treating a stabilized polyolefin fiber with a boron source followed by heating the fiber to a temperature 1000 degrees Celsius or higher in an inert atmosphere so as to convert the stabilized polyolefin fiber in to a carbon fiber.

STATEMENT OF GOVERNMENT INTEREST

This invention was made under a NFE-10-02991 between The Dow ChemicalCompany and UT-Batelle, LLC, operating and management Contractor for theOak Ridge National Laboratory operated for the United States Departmentof Energy. The Government has certain rights in this invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for producing carbon fibers.

2. Introduction

Carbon fibers serve an ever increasing need. The world production ofcarbon fiber in 2010 was 40 kilometric tons (KMT) and is expected togrow to 150 KMT in 2020. Industrial-grade carbon fiber is forecasted tocontribute greatly to this growth, wherein low cost is critical toapplications. The traditional method for producing carbon fibers relieson polyacrylonitrile (PAN), which is solution-spun into fiber form,oxidized and carbonized. Approximately 50 percent of the cost isassociated with the cost of the polymer itself and solution-spinning.

In an effort to produce low cost industrial grade carbon fibers, variousgroups studied alternative precursor polymers and methods of making thecarbon fibers. Precursor alternative to PAN fibers have includedcellulosic yarns, nitrogen-containing polycyclic polymers and evenpitch. Preparing carbon fibers from each different precursor entailschallenges unique to the precursor and the carbonization process foreach precursor has to be designed for the chemistry of the particularprecursor

More recent efforts have included work with stabilized polyolefin (S-PO)fibers such as sulfonated polyethylene fibers. U.S. Pat. No. 4,070,446and WO 92/03601, for example, both teach methods for sulfonation ofpolyethylene fibers and subsequent conversion to carbon fibers and evenfurther conversion to graphitized carbon fibers. The use of S-PO fibersto produce carbon fibers is relatively new technology and historicallyhas produced carbon fibers with lower tensile strength and Young'smodulus compared to carbon fibers from other known precursors. Hightemperature graphitization (typically in excess of 2000 degrees Celsius(° C.)) of S-PO fibers can help increase the resulting carbon fiberYoung's modulus, but also increases the processing cost and complexity.

Work with PAN fibers has revealed that boron can be an effectivecatalyst for graphitizing carbon fibers to increase the fiber modulus.However, as the following references reveal, the required graphitizationtemperature is still quite high even when the fiber includes boroncatalyst. Moreover, the references reveal that boron can actually causea reduction in tensile strength unless heating above 2300° C.

Ya Wen et al., Materials and Design 36, 728-734 (2012) presents datathat demonstrates treating PAN fibers with boric acid treated results inan increase in Young's modulus after heating to temperature greater than1250° C., but the tensile strength of the fibers is reduced unlessheated to temperatures in excess of 2300° C.

GB 1295289 reports that boron can serve as a catalyst for facilitatingrapid graphitization of certain polymer fibers at temperature ranges of1800-3200° C. GB1295289 identifies as suitable precursor fibers PAN,cellulosic and nitrogen-containing polycyclic polymer fibers. The boroncatalyst is shown by examples to produces carbon fibers having anincreased Young's modulus relative to similar fibers prepared withoutboron when PAN fibers are heated in excess of 2200° C.

Other references also identify boron as a suitable graphitizationcatalyst for producing graphite fibers with improved properties providedthe graphitization temperature exceeds 2000° C. See, for example,DE1949830A1, JP3457774B2, JP3303424B2, and Cooper, G A, Mayer R M,Journal of Materials Science 6 (1971) 60-67.

PAN fibers have a different chemical structure from S-PO fiberprecursor. It remains unclear in what way boron would affect conversionof S-PO fibers to carbon fibers, or if it would affect such a conversionat all.

It is desirable to provide a process for creating carbon fibers fromS-PO fibers, such as sulfonated polyolefin fibers, that increasesYoung's modulus and preferably also tensile strength of the resultingcarbon fiber without requiring heating to temperatures in excess of2000° C., or even 1800° C.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a solution to the need for a process forcreating carbon fibers from stabilized polyolefin fibers, such assulfonated polyolefin fibers, that increases Young's modulus andpreferably also tensile strength of the resulting carbon fiber withoutrequiring heating to temperatures in excess of 2000° C., or even 1800°C.

Surprisingly, the present invention is a result of discovering thatboron serves as a catalyst during carbonization of stabilized polyolefinfibers. Without being bound by theory, the boron likely acts as agraphitization catalyst in the S-PO fiber and facilitates graphitizationof the S-PO fiber during carbonizing at temperatures even below 2000° C.and even below 1800° C. What is even more surprising is that boronuniquely affects carbonization of S-PO fibers in a way that results inan increase in both Young's modulus and tensile strength at temperaturesbelow 1800° C. This result is in contrast to how boron catalyzes PANduring carbonization as noted above in the Background. Catalytic effectsof boron in carbonizing S-PO fibers are previously unknown. Similarly,catalytic effects of boron at temperatures below 1800° C. sufficient toincrease both Young's modulus and tensile strength are previouslyunknown.

In a first aspect, the present invention is a process comprisingtreating a stabilized polyolefin fiber with a boron source followed byheating the fiber to a temperature of 1000° C. or higher in an inertatmosphere so as to convert the stabilized polyolefin fiber into acarbon fiber.

The process of the present invention is useful for preparing carbonfibers from polyolefin fibers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-4 illustrate wide angle X-ray scattering analysis data forsulfonated polyolefin fibers of Comparative Example C and Example 6during carbonization.

DETAILED DESCRIPTION OF THE INVENTION

Test methods refer to the most recent test method as of the prioritydate of this document when a date is not indicated with the test methodnumber. References to test methods contain both a reference to thetesting society and the test method number. The following test methodabbreviations and identifiers apply herein: ASTM refers to ASTMInternational (formerly American Society for Testing and Materials); ENrefers to European Norm; DIN refers to Deutsches Institut für Normung;and ISO refers to International Organization for Standards.

“And/or” means “and, or as an alternative”. All ranges include endpointsunless otherwise indicated.

“Elastic modulus” and “Young's modulus” are interchangeable. The processof the present invention is useful for preparing carbon fibers,preferably graphitized fibers from stabilized polyolefin fiber.

“Carbon fiber” is a fiber comprising an excess of 70 wt %, preferably 80wt % or more, still more preferably 90 wt % or more by weight of thefiber and wherein the carbon weight exceed hydrogen weight by a factorof twenty or more, preferably fifty or more.

“Graphite fiber” is a form of carbon fiber this is characterized byordered alignment of hexagonal carbon rings—a crystal-like structure andorder. A carbon fiber becomes more graphite in nature as the amount andorganized arrangement of hexagonal rings increases in the carbon fiber.

“Graphitized carbon fibers” are carbon fibers demonstrating some degreeof crystal-like structure and order.

Stabilized polyolefin (S-PO) fibers are polyolefin fibers that have beenchemically modified so as to experience less than 10 weight-percent (wt%), more preferably less than 5 wt %, or even more preferably less than1 wt % and preferably no detectable hydrocarbon loss at temperatures upto 600° C. by thermogravimetric analysis based on fiber weight.Polyolefin (PO) fibers can be converted into S-PO fibers bycrosslinking, oxidizing (for example air oxidation) or sulfonating thepolyolefin fibers.

The polyolefin fiber that is chemically modified so as to become an S-POcan be polyolefin homopolymer or multipolymer, including multipolymerscomprising both olefins and non-olefins. Herein, “multipolymer” refersto polymers of more than one type of monomer such as copolymers,terpolymers and higher order polymers. Desirably, the polyolefin fiberis a homopolymer or copolymer comprising one or any combination or morethan one of ethylene, propylene, butadiene and/or styrene units.

Polyethylene homopolymer and multipolymers, particularly copolymers, areespecially desirable polyolefin fibers. Preferable polyethylenecopolymers include ethylene/octene copolymers, ethylene/hexenecopolymers, ethylene/butene copolymers, ethylene/propylene copolymers,ethylene/styrene copolymers, ethylene/butadiene copolymers,propylene/octene copolymers, propylene/hexene copolymers,propylene/butene copolymers, propylene/styrene copolymers, propylenebutadiene copolymers, styrene/octene copolymers, styrene/hexenecopolymers, styrene/butene copolymers, styrene/propylene copolymers,styrene/butadiene copolymers, butadiene/octene copolymers,butadiene/hexene copolymers, butadiene/butene copolymers,butadiene/propylene copolymers, butadiene/styrene copolymers, or acombination of two or more thereof.

The polyolefin is desirably a multipolymer, preferably copolymer ofethylene and octene.

Polyolefin multipolymers can have any arrangement of monomer units. Forexample, the polyolefin multipolymer can be linear or branched,alternating in monomer units or blocks of monomer units (such as diblockor triblock polymers), graft multipolymer, branch copolymers, combcopolymers, star copolymers or any combination of two or more thereof.

The polyolefin fiber and S-PO fiber can be of any cross-sectional shapesuch as circular, oval, star-shaped, that of a hollow fiber, triangular,rectangular and square.

The S-PO fibers are desirably sulfonated polyolefin fibers. Sulfonatedpolyolefin fibers are polyolefin fibers that are stabilized by beingsulfonated and comprising sulfate functionalities. Any means ofsulfonating a polyolefin fiber is suitable for preparing the sulfonatedpolyolefin fiber for use in the process of the present invention. Forexample, a suitable means of sulfonating a polyolefin fiber is byexposing the polyolefin fiber to a sulfonating agent such asconcentrated and/or fuming sulfuric acid, chlorosulfonic acid, and/orsulfur trioxide in a solvent and/or as a gas. Preferably, prepare thesulfonated polyolefin fiber by treating the polyolefin fiber with asulfonating agent selected from fuming sulfuric acid, sulfuric acid,sulfur trioxide, chlorosulfonic acid or any combination thereof.Sulfonation can be a step-wise process during which a polyolefin fiberis exposed to a first sulfonating agent and then a second sulfonatingagent and the, optionally, a third and optionally more sulfonatingagents. The sulfonating agent in each step can be the same or differentfrom any other step. Typically, sulfonating occurs by running apolyolefin fiber through one or more than one bath containing asulfonating agent.

One desirable method for sulfonating a polyolefin fiber is to treat thepolyolefin fiber with fuming sulfuric acid (first step), then with aconcentrated sulfuric acid (second step) and then by a secondconcentrated sulfuric acid treatment (third step). The temperatureduring each of the three steps can be the same or different from oneanother. Preferably, the temperature in the first step is lower than thetemperature during the second step. Preferably the temperature duringthe second step is lower than the temperature during the third step.Examples of suitable temperatures include: for the first step: zerodegrees Celsius (° C.) or higher, preferably 30° C. or higher and morepreferably 40° C. or higher and at the same time desirably 130° C. orlower, preferably 100° C. or lower; desirably 105-130° C. for the secondstep and desirably 130-150° C. for the third step. Residence times ineach step can range from 5 minutes or more to 24 hours or less.

Treat the S-PO fiber with a boron source. Suitable boron sources includeboric acid, phenyl boronic acid. It is desirable to use an aqueous boricacid solution as the boron source and to treat a S-PO fiber by exposingthe S-PO fiber to the aqueous boric acid solution. The concentration ofboric acid in the aqueous boric acid solution is typically 0.09 molesper liter (M) or higher, preferably 0.1 M or higher, more preferably 0.2M or higher, 0.3 M or higher, 0.4 M or higher, even 0.5 M or higher.Most preferably the boric acid solution is a saturated boric acidsolution at the temperature of exposure to the S-PO fiber.

It is desirable to expose the S-PO fiber to a sufficient concentrationof boron source for a sufficient period of time so as to incorporatesufficient boron with the S-PO fiber to obtain a boron concentration inthe final carbon fiber as described below with the description of thecarbon fiber.

Heat the S-PO fiber that has been treated with a boron source in aninert atmosphere in order to convert the S-PO fiber into a carbon fiber.Heating in an inert atmosphere prevents oxidative degradation of theS-PO fiber during carbonization. An inert atmosphere contains less than100 parts per million by weight oxygen based on total atmosphere weight.The inert atmosphere can contain inert gasses (gasses that will notoxidize the PO fiber during the heating process). Examples of suitableinert gasses include nitrogen, argon, and helium. The inert atmospherecan be a vacuum, that is a pressure lower than 101 kiloPascals. Theoxygen level can be reduced purely by purging with one or more than oneinert gas, by purging with inert gas and drawing a vacuum, or by drawinga low enough vacuum to reduce the oxygen level to a low enoughconcentration to preclude an undesirable amount of oxidation of the S-POfiber during heating.

Heat the S-PO fiber in an inert atmosphere to a temperature of 1000° C.or higher in order to carbonize the S-PO fiber. Preferably, heat theS-PO fiber in an inert atmosphere to a temperature of 1150° C. orhigher, more preferably 1600° C. or higher, still more preferably 1800°C. or higher. Heating can be to a temperature of 2000° C. or higher,2200° C. or higher, 2400° C. or higher and even 3000° C. or higher.However, generally heating is to a temperature of 3000° C. or lower.Higher heating temperatures are desirable for carbonizing S-PO fibersbecause higher temperature can convert the fiber to a graphite fiberhaving higher strength, higher Young's modulus or both than non-graphitecarbon fiber. One of the surprising results of the present invention isthat graphitization (that is, formation of crystal structure in a carbonfiber) can be achieved from a S-PO fiber by heating to only 1800° C. orlower. That is, the present invention provides for graphitization of aS-PO fiber without heating the S-PO fiber to temperatures above 1800° C.

Heat the fiber as long as necessary to achieve desired properties.Generally, the longer a fiber is heated the more complete thecarbonization and more aligned the carbon becomes. Generally, theduration of heating is a balance of processing the fibers fast enough tobe commercially viable while still heating long enough to achievedesired fiber properties.

After heating the S-PO fiber and converting it to a carbon fiber it isdesirable for the carbon fiber to have a boron concentration of at least0.3 mole-percent (mol %), preferably 0.35 mol % or more, more preferably0.5 mol % or more, yet more preferably one mol % or more, even morepreferably 2.5 mol % or higher, still more preferably 2.8 mol % orhigher, yet more preferably 3 mol % or higher, still even morepreferably 3.3 mol % or higher and even yet more preferably 3.6 mol % orhigher. Typically, the concentration of boron is 10 mol % or less, moretypically 5 mol % or less in the final carbon fiber. Boron concentrationis relative to the total moles of elements in the carbon fiber.Determine boron concentration in the carbon fiber by inductively coupledplasma (ICP) analysis according to the method set forth below in theExamples section.

The present invention is a result of discovering that treating a S-POfiber with boron prior to carbonization allows production of carbonfibers having higher strength, higher Young's modulus or both higherstrength and higher Young's modulus than carbon fibers produced fromS-PO fibers that do not contain boron and that are carbonized at thesame carbonization temperature. For the sake of this invention strengthrefers to tensile strength. Characterize tensile strength and Young'smodulus according to ASTM method C1557.

EXAMPLES

Melt-spin a polyethylene/1-octene copolymer (melt index of 30; densityof 0.9550 grams per milliliter; polydispersity of 3.0) into a continuousfiber tow containing 1700 filaments (tenacity of 4.4 grams per denier;elongation to break of 8.4%; diameter of 8.2 microns). Sulfonate thefiber tow in a 4-bath continuous process under 25 megaPascals (MPa)tension in the first bath and 15 MPa tension in the subsequent baths.Feed the fiber tow at a rate that corresponds to a residence time ofapproximately 60 minutes in each bath. The first bath is 20 mole-percent(mol %) fuming sulfuric acid at 50° C. The second bath is 96 mol %sulfuric acid at 120° C. The third bath is 96 mol % sulfuric acid at140° C. The fourth bath is deionized water for Comparative Examples Aand B, aqueous boric acid (BA) solution of various concentrations forExamples 1-5 and 0.082 molar aqueous phenyl boronic acid (PBA) forExample 5. From the fourth bath, spool the fiber.

Note, Comparative Example B and Example 5 are made from a differentsample of PO fiber on a different day than Comparative Example A andExamples 1-4. Therefore, comparisons of results are most accurate whencomparing Comparative Example A and Examples 1-4 and Comparative ExampleB with Example 5.

Carbonize the sulfonated fiber by passing a 10 centimeter (four inch)sample through a continuously nitrogen-purged three-zone carbonizationfurnace with heated zone temperatures of 650° C., 950° C. 1150° C. Runthe sample fibers through the furnace with 5.5 MPa tension and for atotal resonance time of 14 minutes. For carbonizations of 1200° C. andhigher (see Table 1), further pass the sample through a continuouslynitrogen purged single zone KYK furnace with 5.5 MPa tension and a totalresonance time in the hottest zone of 2.5 minutes.

Resulting strength and Young's modulus values for the resulting carbonfibers for the Examples are in Table 1 given in units of gigaPascals(GPa). Concentration of boron in the resulting carbon fiber is given forselect examples in mol % based on fiber mole composition as determinedby ICP analysis.

Conduct ICP analysis using the following procedure. Prepare samples byacid digestion using single reaction chamber microwave digestiontechnology with a Milestone UltraWave digestion system. Transferapproximately 10 milligrams of carbon fiber into a quartz digestion tubeand add 0.5 milliliters of high purity deionized water rand twomilliliters of concentrated nitric acid. Pre-pressurize the reactionchamber with nitrogen at 4 mega Pascals (40 bar) and heat samples to200-250° C. with microwave energy to perform the digestion. Afterdigestion, dilute the sample to 15 milliliters with high puritydeionized water. Analyze the sample using an inductively coupled plasmaemission spectrometer (ICP-OES). Calibrate the ICP-OES suing dilutionsof a certified aqueous boron standard over a range of approximately 1-10micrograms/gram and a 5% nitric acid blank. Prepare the calibrationstandards and samples based on a weight basis. Run the samples againstthis calibration. Numerous rinses and calibrations are done along withquality control checks to ensure an absence of carry over throughout themeasurement run due to the tendency of boron to cause memory effects.Accuracy was confirmed by spiking a fiber sample containing no addedboron with a certified aqueous boron standard and doing the analysis.

TABLE 1 Example A 1 2 3 4 B 5 Bath 4 Composition DI 0.1M 0.3M 0.5MSaturated DI 0.082M Water BA BA BA BA Water PBA 1150° C. CarbonizationYoung's Modulus 84 78 67 75 67 80 54 (GPa) Strength (GPa) 1.25 1.35 0.911.2 1.32 1.3 0.97 Boron (mol %) 0 1.0 1.9 2.8 3.6 0 NM* 1600° C.Carbonization Young's Modulus 101 95 95 139 143 93 125 (GPa) Strength(GPa) 1.66 1.33 0.99 1.44 1.60 1.37 1.4 1800° C. Carbonization Young'sModulus 116 131 167 201 203 108 188 (GPa) Strength (GPa) 1.25 1.4 1.372.4 1.97 1.55 1.73 Boron (mol %) 0 0.35 2.0 3.3 3.6 NM NM 2000° C.Carbonization Young's Modulus -broke- 175 225 283 264 193 224 (GPa)Strength (GPa) -broke- 1.6 1.56 2.0 1.22 2.2 2.23 2200° C. CarbonizationYoung's Modulus 153 208 302 351 278 NM NM (GPa) Strength (GPa) 1.5 1.61.6 1.3 0.74 NM NM 2400° C. Carbonization Young's Modulus 195 254 353406 -broke- NM NM (GPa) Strength (GPa) 1.89 2.0 1.7 1.3 -broke- NM NM*NM = not measured.

The data of Table 1 reveals measurable increases in Young's modulusand/or strength of a carbonized sulfonated PO fiber that has beencarbonized at temperatures ranging from 1150° C. to 2400° C. Increasesin Young's modulus and strength are evident an a broad range of borontreatment concentration and with a broad range of boron concentrationsin the resulting carbon fiber.

Prepare pre-carbonized fibers for Comparative Examples B and 6 in likemanner as Comparative Example A and Example 3, respectively except use45.7 centimeter (18 inch) fiber samples instead of 10 centimeter (fourinch) fiber samples. Analyze Comparative Example B and Example 6 bywide-angle X-ray diffraction (WAXD) while carbonizing under a heliumatmosphere. Maintain the fibers under 163 MPa tension during the WAXDanalysis and conduct the analysis while heating the fibers to determinechange in crystal structure with heating.

WAXD characterization for cos²φ, Lc and d₀₀₂ is in FIGS. 1-4respectively. FIG. 1 illustrates that the degree of orientation in thefiber undergoes a greater degree of orientation at lower temperatureswhen boron is present. FIG. 2 further illustrates that the length ofcrystallites is greater at lower temperatures when boron is present.FIG. 3 illustrates that the spacing between crystal layers is smaller atlower temperatures (therefore, the crystal structure is more pure) atlower temperatures when boron is present. FIG. 4 illustrates that thenumber of crystal sheets is higher at lower temperatures when boron ispresent. The WAXD data confirms that graphitization is more extensive atlower temperatures in boron treated sulfonated PO fibers than inboron-free sulfonated PO fibers.

What is claimed is:
 1. A process comprising treating a stabilizedpolyolefin fiber with a boron source followed by heating the fiber to atemperature of 1000° C. or higher in an inert atmosphere so as toconvert the stabilized polyolefin fiber into a carbon fiber.
 2. Theprocess of claim 1, further characterized by the stabilized polyolefinfiber being a sulfonated polyolefin.
 3. The process of claim 1, furthercharacterized by the polyolefin being a copolymer of ethylene andoctene.
 4. The process of claim 1, further characterized by graphitizingthe stabilized polyolefin fiber without heating to temperatures greaterthan 1800° C.
 5. The process of claim 1, further characterized by thetreating of the stabilized polyolefin fiber with a boron sourceincluding subjecting the stabilized polyolefin fiber to an aqueous boronsource.
 6. The process of claim 1, further characterized by the boronsource comprising boric acid.
 7. The process of claim 1, furthercharacterized by treating the stabilized polyolefin fiber with a boronsource by submersing the stabilized polyolefin fiber into aqueous bathof boric acid at a boric acid concentration of 0.1 moles per liter ormore.
 8. The process of claim 1, further characterized by disposingsufficient boron on the stabilized polyolefin fiber so that theresulting carbon fiber has a boron concentration of one mole-percent ormore.